The present invention relates to the preparation of aluminium alloys useful in automotive metal working applications, and particularly relates to the provision of alloys possesssing improved resistance weldability.
Certain aluminum alloys have long been useful in applications such as the aircraft industry where extended sheets of material are joined to prepare a vehicle structure or body by resistance spot welding. Particularly in the aircraft industry, the technique of resistance spot welding has been highly developed where costs of the required equipment and elaborate production controls can be justified.
Two physical properties of aluminum are important in the practice of resistance spot welding. The first of these properties is the electrical resistivity which is known to be low by comparison with well-known steels, and therefore requires high welding currents to generate enough heat to make the weld. The other property comprises contact resistance existing at the surface of the metal, which, if excessively high and variable, causes pick-up or sticking of the metal to the welding electrodes, as well as extreme variability in the size, shape and strength of the resulting weld.
With the recent concern over energy consumption, the automotive industry has reviewed the types of materials being employed in substantial amounts in the structure of motor vehicles to determine whether efficiencies in weight reduction can be achieved by the selection of alternate materials. As the result, aluminum sheet alloys, which have characteristically found use in the aircraft industry, as well as alloys with more moderate strength and greater formability, are being investigated because of their reduced weight, corrosion resistance, and other favorable properties. An important factor, however, in the consideration of aluminum is its amenability to the resistance welding techniques presently useful with the steels currently employed. Thus, the ease of resistance welding in terms of minimal control and lower current requirements comprises an important factor which renders it desirable to tailor aluminum alloys to exhibit improved resistance weldability. One of the observations that has been made is that an increase in the bulk resistivity of the metal appears necessary, as it reduces total current requirements and makes contact resistances less critical.